1. Field of the Invention
The principles of the present invention are directed to a particle measurement system, and more particularly, but not by way of limitation, to a particle sensor having a single component collecting system.
2. Description of Related Art
Most people are familiar with the sight of dust in a sunbeam. Four things that are necessary for this include: sunlight (to illuminate the dust), dust (to reflect the sunlight), air (to carry the dust), and a person's eye (to see the dust, or more specifically to see the light reflected by the dust). An optical particle counter uses the same principles, but refines them to maximize its effectiveness. In modern particle counters, a laser light source is typically used, the sample (e.g., air) is controlled, and a high sensitivity photo or light detector is utilized to detect light that particles scatter. Typical uses of an optical particle counter include clean rooms, hospitals, and other facilities where cleanliness is important.
It is important to distinguish the science of particle counting from other scientific fields, such as photometry and cytometry, which also utilize scattered light, but in which the density of the particles in the fluid is relatively large; often it is the particles of the fluid itself that are detected and analyzed. These latter systems rely on collecting scattered light from thousands, millions, and even billions of particles; therefore, their principles of operation are very different from the principles used in particle counters, which detect individual particles suspended in a fluid.
As understood in the art, particle counters do not directly count particles, but rather count flashes of light scattered by particles (or shadows cast by backlit particles). Also, particle counters do not count every particle in a system of interest, but rather generate a statistically valid sample representative of the number of particles for average fluid in the rest of the system.
A large portion of the aerosol micro-contamination market having 0.3 micrometer (μm) to 0.5 μm sensitivity, 0.1 cubic feet per minute (CFM) to 1.0 CFM sample flow rate, use low cost particle sensors for determining particle counts. These sensors generally utilize a low power laser diode, typically 25–50 milliwatts (mW), a PIN photo-diode, and a single component light collecting system, which typically includes a surface spherical mirror. A single component collecting system is generally utilized to minimize system size and cost. While a single component collecting system cannot achieve the image blur-size capabilities of a multi-component solution, it is capable of collecting light at a relatively broad collection angle, typically +/−50° to 60°, and directing light onto a light detector. The image plane typically includes a photo-diode or other light detector. Magnification of a single component collecting system is generally slightly larger than 1×. One limitation of the single component collecting system is the inherent inability to effectively reduce optical noise known as the scattered light noise floor of the system.
Unlike multiple component collecting systems that have the ability to remove the laser aperture assembly from direct view of the photo-diode, a single component system has limited options when dealing with scattered light. All conventional particle measurement systems utilizing a single component collecting system include an aperture assembly that is in direct view of the system light detector. FIG. 1 is a schematic illustrating an exemplary particle measurement system 100 that includes a single component light collecting system 102. The particle measurement system 100 includes a housing 104 that houses the single component light collecting system 102 and electronics (not shown) used to count the number of particles detected by the single component light collecting system 102. The single component light collecting system 102 includes a sample chamber 106 composed of a frame 108, which maybe formed of multiple members. The frame 108 may define multiple apertures 110, 112, and 114. A pair of the apertures 110 and 112 may be configured in opposing relation. A laser diode module 116 including a laser diode (not shown) may be coupled to the frame 108 or another structural component located in the housing 104 and be positionally aligned to direct a light beam 118 into the sample chamber 106 via aperture 110. In the case of using a laser diode, the light beam 118 is a laser beam. Alternatively, if the light source is a non-lasing light source, then the light beam 118 is not a laser beam.
An aperture assembly 120 is typically configured as an aperture tube 119 that engages the aperture 110. The aperture assembly 120 is utilized to collimate the light beam 118 to minimize divergence of the light beam in the sample chamber so as to reduce optical noise. A beam stop 122 may be coupled to the frame 108 in optical alignment with the aperture 112. The beam stop 122 functions to absorb laser light that has exited the sample chamber 106.
An inlet orifice 124 may be coupled to the frame 108 and be utilized to flow air into the sample chamber 106 and through the light beam 118. A collecting mirror 126 or other reflecting device may be coupled to the frame 108 on a first side relative to the light beam 118. A light detector 128 may be disposed in relation to the aperture 114. In one embodiment, the light detector 128 is a photo diode. More specifically, the light detector may be a PIN photo diode. The light detector 128 may be coupled to the frame 108 or another member in the housing 104. Detector 128 is electrically coupled to a signal processor/amplifier 129. The signal processor/amplifier produces a system output signal on output 131.
In operation, the laser diode module 116 generates a light beam 118 that passes into the sample chamber 106 via the aperture assembly 120, and the sample chamber 106, through the aperture 112, and into the beam stop 122. The inlet orifice 124 flows air to be sampled through the light beam 118, such that particles in the air reflect light from the light beam 118 either directly into the light detector 128 or into the collecting mirror 126. The collecting mirror 126, which may be spherically shaped to focus light onto the light detector 128, reflects the light into the light detector for measurement thereby. In response to incident light, the light detector 128 generates an output signal, preferably a voltage pulse, characteristic of one or more parameters of the particles, as the size of the particles. The detector output signal is processed and/or amplified by signal processor/amp 129 to produce an output signal on output 131 that is characteristic of one or more parameters of the particles, such as size and number of particles in a size range.
Scattered light is generally caused by spontaneous emission of light from a facet (not shown) of the laser diode module 116. Laser light generated from the facet is considered to be stimulated emission light and is coherent. The stimulated emission light is predictable and is shaped and imaged by lenses of the laser diode module 116. The spontaneous emission light generated by the facet is not coherent and cannot be successfully shaped and imaged by lenses of the laser diode module 116. This spontaneous emission light causes a fairly broad light pattern that is centered on the more tightly focused primary laser beam. The unwanted spontaneous emission light pattern may reach over one percent of the power level of the light beam 118.
Collected light energy from a 0.3 micrometer particle may be as small as 0.000,015 percent of the power level of the light beam 118. In such a system, the unwanted spontaneous emission light pattern represents over 60,000 times the light level of the particle of interest, and therefore, is to be minimized and controlled. All conventional particle sensors or particle measuring systems utilize either an aperture tube, one or more apertures, or a combination of both to minimize the spontaneous emission light that is allowed to enter the sample chamber 106 of the particle measurement system 100.
The aperture assembly 120 attempts to prevent as much spontaneous emission light as possible from entering the sample chamber 106. The remaining portion of this spontaneous emission light that cannot be blocked is then collimated as well as possible by the aperture assembly 120. Ideally, this somewhat collimated light is then funneled through the optics chamber while minimizing the amount of light that diverges enough to come in contact with any physical structure of the sample chamber 106. Any light that diverges enough to contact the physical structure of the sample chamber 106 then scatters light energy that could be detected by the light detector 128. To minimize the amount of divergence, the aperture assembly 120 is sized to be as close to the viewed sample volume (e.g., air injected into the sample chamber 106) as possible. This inherently places the aperture assembly 120 in direct view of the light detector 128. Unfortunately, the aperture assembly 120 inherently produces unwanted diffraction and reflection light patterns of their own due to the spontaneous emission light reflecting off of an inside surface of the aperture assembly 120.
Once spontaneous emission light enters the sample chamber 106 and is detected by the light detector 128, it is referred to as scattered light noise. The fundamental noise limit of conventional single component particle measurement systems is scattered light noise. As shown in FIG. 1, the light detector 128 has a direct view of the aperture assembly 120 within the view angle 130, shown as dashed lines.
Conventional single component particle measurement systems have been designed to reduce the scattered light noise in a number of ways, including by making the inside of the aperture assembly 120 and sample chamber 106 light absorbing. In doing so, the surfaces of the aperture assembly 120 and sample chamber 106 are bead blasted and then either anodized or painted with a flat black paint. This bead blasting and flat black paint treatment causes the surfaces to appear multi-dimensional to the spontaneous emission light and very absorbing by scattering the light off of the peaks and valleys of the surfaces, and thus hitting the surfaces many times to have a better chance of being absorbed and not detected.
However, even by treating the inner surfaces of the aperture assembly 120 and sample chamber 106, complete elimination of the scattered light noise is not possible so that power from the laser diode module is increased to raise the signal-to-noise ratio. The signal-to-noise ratio goes up as the square root of the laser power, so by using a more powerful laser, the signal-to-noise ratio is increased. Another technique used to reduce the scattered light noise includes using a light dam around the photo diode. While the light dam eliminates some of the scattered light noise, it blocks some light from the particles in the light beam 118, thereby requiring more power to drive the laser diode to overcome the blockage. This increased power causes the laser diode to have a shorter lifespan, which is ultimately more costly for the manufacturer of the particle measurement system 100. The use of these techniques, even combined, still results in the scattered light noise being the primary noise factor in the optical system of the particle measurement system 100.